Enhancing the usability of carrier phase measurements

ABSTRACT

An orientation of a mobile first antenna is determined based at least on sensor information. Further, a computational compensation of a difference between the orientation of the first antenna and an orientation of a second antenna is caused, for reducing an influence of this difference on calculations using carrier phase measurements of satellite signals received by the first antenna and the second antenna.

FIELD OF THE INVENTION

The invention relates to enhancing the usability of carrier phasemeasurements of satellite signals, for example in the scope of arelative positioning.

BACKGROUND OF THE INVENTION

An absolute positioning of a device is supported by various GlobalNavigation Satellite Systems (GNSS). These include for example theAmerican Global Positioning System (GPS), the Russian Global NavigationSatellite System (GLONASS), the future European system Galileo, theSpace Based Augmentation Systems (SBAS), the Japanese GPS augmentationQuasi-Zenith Satellite System (QZSS), the Locals Area AugmentationSystems (LAAS), and hybrid systems. The satellites of these systems arealso referred to as space vehicles (SV).

The constellation in GPS, for example, consists of more than 20satellites that orbit the earth. Each of the satellites transmits twocarrier signals L1 and L2. One of these carrier signals L1 is employedfor carrying a navigation message and code signals of a standardpositioning service (SPS). The L1 carrier phase is modulated by eachsatellite with a different C/A (Coarse Acquisition) code. Thus,different channels are obtained for the transmission by the differentsatellites. The C/A code is a pseudo random noise (PRN) code, which isspreading the spectrum over a 1 MHz bandwidth. It is repeated every 1023bits, the epoch of the code being 1 ms. The carrier frequency of the L1signal is further modulated with navigation information at a bit rate of50 bit/s. The navigation information comprises inter alia ephemeris andalmanac parameters. Ephemeris parameters describe short sections of theorbit of the respective satellite. Based on these ephemeris parameters,an algorithm can estimate the position of the satellite for any timewhile the satellite is in the respective described section. The almanacparameters are similar, but coarser orbit parameters, which are validfor a longer time than the ephemeris parameters. The navigationinformation further comprises for example clock models that relate thesatellite time to the system time of GPS and the system time to theCoordinated Universal Time (UTC). A GPS receiver of which the positionis to be determined receives the signals transmitted by the currentlyavailable satellites, and it detects and tracks the channels used bydifferent satellites based on the different comprised C/A codes. Then,the receiver determines the time of transmission of the code transmittedby each satellite, usually based on data in the decoded navigationmessages and on counts of epochs and chips of the C/A codes. The time oftransmission and the measured time of arrival of a signal at thereceiver allow determining the pseudorange between the satellite and thereceiver. The term pseudorange denotes the geometric distance betweenthe satellite and the receiver, which distance is biased by unknownsatellite and receiver offsets from the GPS system time.

In one possible solution scheme, the offset between the satellite andsystem clocks is assumed known and the problem reduces to solving anon-linear set of equations of four unknowns (3 receiver positioncoordinates and the offset between the receiver and GPS system clocks).Therefore, at least 4 measurements are required in order to be able tosolve the set of equations. The outcome of the process is the receiverposition.

Similarly, it is the general idea of GNSS positioning to receivesatellite signals at a receiver which is to be positioned, to measurethe pseudorange between the receiver and the respective satellite andfurther the current position of the receiver, making use in addition ofestimated positions of the satellites. Usually, a PRN signal which hasbeen used for modulating a carrier signal is evaluated for positioning,as described above for GPS.

In a further approach known as Real Time Kinematics (RTK), the carrierphases and/or the code phases measured at two GNSS receivers areevaluated for determining the distance and attitude between the tworeceivers very accurately, typically at cm- or even mm-level accuracy.The combination of the distance and attitude between two receivers isalso referred to as baseline. The carrier phase measurements that areperformed at GNSS receivers for an RTK positioning may be exchanged inreal-time or be stored for a later exchange known as post-processing.Usually, one of the GNSS receivers is arranged at a known location andcalled reference receiver, while the other receiver is to be positionedwith respect to the reference receiver and called user receiver orrover. The determined relative position can further be converted into anabsolute position, if the location of the reference position isaccurately known. However, the RTK calculations actually require thatthe positions of both receivers are known at least approximately. Thesepositions can be obtained from determined pseudoranges. Alternatively,it would also be sufficient to know only a reference locationapproximately, since the rover location can be obtained therefrom byadding the baseline estimate to the reference location.

A satellite signal is distorted on its way from a satellite to areceiver due to, for instance, multipath propagation and due toinfluences by ionosphere and troposphere. Moreover, the satellite signalhas a bias due to the satellite clock bias. All errors that are commonto a signal in both receivers can be assumed to correlate between thereceivers and satellites, and thus to vanish in double differencing.

The relative positioning may thus be based more specifically on signalmeasurements at two GNSS receivers, which are used to form doubledifference observables. Such signal measurements may include for examplecarrier phase measurements and PRN code measurements, etc. A doubledifference observable relating to the carrier phase is the difference inthe carrier phase of a specific satellite signal at both receiverscompared to the difference in the carrier phase of another satellitesignal at both receivers. A double difference observable relating to thePRN code may be obtained correspondingly. The double differenceobservables can then be employed for determining the position of thereceivers relative to each other at high accuracy.

With conventional GNSS positioning, two GNSS receivers are able todetermine their location, and therefore the baseline between them, withan accuracy of 5 to 20 meters. The RTK approach, in contrast, allowsdetermining the baseline with a much higher accuracy of 0.1 to 10 cm. Itis noteworthy that this accuracy can be achieved with standardcommercial GNSS-receivers.

When using the RTK approach, however, it has to be considered that acode or carrier phase measured at two receivers is based on differentnumber of whole cycles of the carrier. This effect is referred to asdouble-difference integer ambiguity, which has to be solved. Thisprocess is also called integer ambiguity resolution or initialization.

The double-difference integer ambiguity may be resolved by gatheringcarrier and/or code phase data from a sufficient number of satellites atsufficient measurement instants. The solution may be obtained usingindividual epochs or as a continuous process using filters.

Once the baseline has been determined and the integer ambiguity beenresolved, the integer ambiguity solution may be validated in order todetermine whether it can be relied on. Integer ambiguity validation istypically done using statistical tools.

The solved and validated integer ambiguities may then be used fortracking the baseline between the receivers at high precision, forinstance with a sub-cm accuracy.

Originally, RTK positioning was only available for geodesic surveyingand other applications requiring a high accuracy. The equipment requiredfor such applications is expensive and meant, therefore, only forprofessional use. In these cases, the baseline is moreover oftendetermined off-line. However, it is also possible to obtain ahigh-precision baseline using two low-cost GNSS-enabled handsets, forexample terminals with integrated GNSS-receiver or terminals equippedwith an external Bluetooth GNSS-receiver. The data between the terminalscan be exchanged using any kind of data transfer technology, likegeneral packet radio service (GPRS), wireless local area networks (WLAN)or Bluetooth™. This allows the baseline to be determined and updated inreal-time. This approach is also called mobile Real-Time Kinematics(mRTK), indicating that mobile technology is used to expand the RTK usecases and bring the benefits of the technology to a wider audience.

SUMMARY

The invention proceeds from the consideration that while common errorsto satellite signals received by different antennas are canceled out indouble differencing, additional errors are introduced by the receivingevent, that is, on the path from the surface of the antenna to the feedcable or feed path. Of these errors, only those that are common to allthe satellites cancel out in double differencing, like, for instance,receiver noise and receiver time bias.

In addition, however, an antenna having an anisotropic complex frequencyresponse may generate an error in the measured carrier phase of areceived signal. This error is dependent on the direction of thesatellite with respect to the antenna axis. This means that depending onthe receiving direction of the satellite signal, a bias is induced onthe carrier phase observable. This bias has an effect on relativepositioning computations, in case it is not the same at both receivers.If the bias difference is large, meaning in the order of tens ofdegrees, the baseline determination may fail. Even if a baselinedetermination is possible, it may be unreliable due to the systematicerrors in the carrier phase observables.

The problem is of particular relevance when the antenna is integrated ina mobile device, in which case the antenna may have an arbitraryorientation.

High-quality antennas may have a fairly isotropic phase response. Withsuch antennas, errors in the carrier phase measurements can be avoided.However, high-quality antennas are expensive. Thus, they are suitedmainly for professional use cases. In particular in mobile terminals,the antenna solutions are often sub-optimal, and the phase responses maybe highly anisotropic.

With such sub-optimal antennas, errors in the carrier phase measurementscan be minimized, if a user is required to set the antennas of bothdevices to a predetermined orientation. When the two antennas arealigned in the same direction, the phase errors are similar to bothreceivers and the errors cancel out in the double differencing process.However, in mobile relative positioning applications, a user may notalways have access to both antennas.

A method is described, which comprises determining an orientation of amobile first antenna based at least on sensor information. The methodfurther comprises causing a computational compensation of a differencebetween the orientation of the first antenna and an orientation of asecond antenna for reducing an influence of this difference oncalculations using carrier phase measurements of satellite signalsreceived by the first antenna and the second antenna.

The expression ‘causing a computational compensation’ is to beunderstood such that the computational compensation is either performedor that instructions for such a computational compensation are provided.

Moreover, an apparatus is described, which comprises a processingcomponent configured to determine an orientation of a mobile firstantenna based at least on sensor information. The processing componentis further configured to cause a computational compensation of adifference between the orientation of the first antenna and anorientation of a second antenna for reducing an influence of thisdifference on calculations using carrier phase measurements of satellitesignals received by the first antenna and the second antenna.

The processing component can be implemented in hardware and/or software.It may be for instance a processor executing software program code forrealizing the required functions. Alternatively, it could be forinstance a circuit that is designed to realize the required functions,for instance implemented in a chipset or a chip, like an integratedcircuit. The described apparatus can be for example identical to thecomprised processing component, but it may also comprise additionalcomponents. The apparatus could further be for example a module providedfor integration into an electronic device, like a wireless communicationdevice or a GNSS accessory device.

Moreover, an electronic device is described, which comprises thedescribed apparatus and in addition the first antenna. Such anelectronic device could be for instance a mobile terminal, an accessorydevice for a mobile terminal or a satellite receiver, etc. Theelectronic device could comprise in addition an interface enabling acommunication with another electronic device.

Moreover, an arrangement is described, which comprises a first deviceincluding the described apparatus and in addition a second deviceincluding the first antenna. The first device could be for example amobile terminal and the second device an accessory device for the mobileterminal, etc.

Moreover, a system is described, which comprises a first deviceincluding the described apparatus and a second device comprising thementioned second antenna. The first device could be for example a mobiledevice like a mobile terminal, an accessory to a mobile terminal or asatellite receiver, etc. The second device could be for example anothermobile device, like a mobile terminal, an accessory to a mobile terminalor a satellite receiver, or a fixed device, like a base station, anetwork server or a local measurement unit, etc.

Finally, a computer program product is described, in which a programcode is stored in a computer readable medium. The program code realizesthe described method when executed by a processor. The computer programproduct could be for example a separate memory device, or a memory thatis to be integrated in an electronic device.

The invention is to be understood to cover such a computer program codealso independently from a computer program product and a computerreadable medium.

Knowing now the antenna response, that is, the complex frequencyresponse function as a function of azimuth and elevation angles withrespect to the antenna axis, allows for compensating the phase offsetsinduced by the anisotropic antenna response.

The invention thus provides a possibility of compensating for undesired,non-common phase offsets in carrier phase measurements from the signalsreceived by two antennas, of which at least one may be non-optimal.Using sensor information for determining the orientation of the firstantenna enables such a compensation as well for a mobile first antenna.

As a result of the compensation, cheaper antennas having a lower qualitycan be utilized without negative effect on calculations using thecarrier phase measurements.

In one exemplary embodiment, at least two different options are providedfor reducing an influence of a difference between the orientation of thefirst antenna and the orientation of the second antenna on calculationsusing carrier phase measurements of satellite signals received by thefirst antenna and the second antenna. One of the options can then beselected. In case a first one of the options is selected, the abovementioned computational compensation of a difference between theorientation of said first antenna and the orientation of the secondantenna is caused. In case a second one of the options is selected, analignment of the antennas is initiated to obtain equal orientations ofthe first antenna and the antenna.

There may be various criteria for selecting the first or the secondoption.

In one exemplary embodiment, the second option is selected in case thefirst antenna and the second antenna receiving satellite signals aresuited to be aligned, while the first option is selected otherwise.

For supporting the second option, an indication of a type of the secondantenna may be received. It may then be determined that the firstantenna and the second antenna are suited to be aligned in case thefirst antenna and the second antenna are of a same type and an alignmentof at least one of the antennas is supported. If identical antennas arealigned, the antenna phase response asymmetry is removed in doubledifferencing.

In another exemplary embodiment, in general the first option may beselected. The second option may then be selected only in case acomputational compensation has not been successful. If the type of theantennas is known, the second option might be selected only in caseidentical antennas are concerned, because otherwise, even with alignedantennas the antenna phase response asymmetry cannot be expected to beremoved in double differencing.

Initiating an alignment may comprise instructing a user to align thefirst antenna. A first antenna in a mobile terminal, for instance, mayoften be considered a compromise. While a car can have a patch place ona large ground plane, which is an omni-directional horizontal optimum, aterminal tends to have a much smaller antenna construction, which meansthat the mass of the ground plane is the dominant factor. Most usershold a mobile terminal vertical or near vertical, which will not providean omni-directional response or the best phase response to all possiblepoints of transmission in the sky. In such a case, in which apredominately horizontal antenna type is used, the simplest instructionto a user could thus be to lay or hold the mobile terminal horizontally.It is to be understood, though, that various other instructions may beimplemented as well.

Alternatively, initiating an alignment may comprise requesting anotherdevice to cause an alignment of the second antenna. The other device maythen cause an alignment for instance by selecting one of severalavailable antennas as the second antenna, by steering the second antennato assume an orientation similar to the orientation of the first antennaor by instructing a user to align the second antenna. It is alsopossible to combine several approaches. For example, a user could firstbe instructed to align the first antenna, and another device could thenbe requested to cause an alignment of the second antenna to compensatefor a remaining deviation between the orientations of the first antennaand the second antenna.

According to a further embodiment, another device may also be requestedto select one of different available antennas as the second antenna suchthat the second antenna is of the same type as the first antenna. Theother device may, for example, have access to antennas of differentcommon types which belong to one or more antenna arrays.

It is to be understood that receiving an indication of an orientation ofa first antenna receiving satellite signals and causing an alignment ofa second antenna receiving satellite signals in accordance with theindication of an orientation of the first antenna and/or selecting anantenna of an indicated type can also be considered as an independentapproach that may be implemented separately in a device.

The orientation of the first antenna could be determined in a globalcoordinate system. This could be sufficient, if the first antenna andthe second antenna are of the same type. Alternatively or in addition,determining the orientation of the first antenna may comprisedetermining the orientation relative to a path of satellite signalsreceived by the first antenna. This can be achieved by taking intoaccount not only sensor information but in addition information on thecurrent position of satellites from which signals are received.Satellite positions can be calculated in a global coordinate systemusing ephemeredes from satellite signals or from assistance data. Thepositions can then be converted to the antenna coordinate system byusing the antenna orientation information, which is available fromsensor information.

Causing a computational compensation may comprise correcting the carrierphase measurements themselves by canceling a phase offset.

It would be possible, for instance, to cancel a phase offset in carrierphase measurements of satellite signals received by the first antennacompared to carrier phase measurements of the satellite signals receivedby the first antenna that would result with a predetermined orientationof the first antenna. This approach can be used for instance in case theorientation of the second antenna is fixed and corresponds to thepredetermined orientation.

Alternatively, it would be possible to cancel a phase offset in carrierphase measurements of satellite signals received by the first antennacompared to carrier phase measurements of the satellite signals receivedby the first antenna that would result if the orientation of the firstantenna was equal to the orientation of the second antenna. Thisapproach allows taking account as well of variable orientations of thesecond antenna.

In addition, phase offsets caused by different antenna types can betaken into account in the compensation. To this end, antenna carrierphase patterns for different antenna types may be considered.

In case the same entity, which determines the orientation of the firstantenna, performs the computational compensation or causes thealignment, it could receive to this end an indication of the orientationof the second antenna.

Compensated carrier phase measurements could be used in calculating arelative position between the first antenna and the second antenna. Therelative positioning could be based forming double-differences andinteger ambiguity resolution. With the compensation, the baselineaccuracy is improved, because a significant error source is removed.Moreover, the integer ambiguity resolution process becomes morereliable, since double difference measurements become non-biased. Theresolution process assumes that the double-difference observables arenormally distributed and non-biased. However, the phase error from theanisotropic phase response results in the violation of this assumptionand leads to failures in the resolution. This problem is alleviated withthe described approach.

In case causing the computational compensation comprises correctingcarrier phase measurements of signals received by the first antenna, thecorrected carrier phase measurements could also be transmitted to adevice having access to carrier phase measurements of the second antennafor a relative positioning between the first antenna and the secondantenna.

It is to be understood that it is not required that the carrier phasemeasurements themselves are corrected by the computational compensation.The compensation could also be realized in the scope of the calculationsusing the carrier phase measurements. For example, in casedouble-differences are formed from the carrier phase measurements for arelative positioning, causing the computational compensation couldcomprise correcting these double differences.

New satellite systems bring about new frequencies in addition to currentGPS L1, which further improves the mRTK capabilities. However, the newfrequencies also introduce the need to compensate for the phase errorsbetween signals at different frequencies, although they might bereceived with the same antenna. For example, if GPS L1 and L2 are usedto form the wide lane observable, the phase offset between L1 and L2frequencies should be compensated for. This is because the phaseresponse is also a function of frequency.

The entity causing or performing an alignment and/or a computationalcompensation could therefore also receive an indication of the type ofthe satellite signals received by the second antenna. This indicationcould be used as an additional criterion for selecting one of theavailable options. Further, this indication could be used for causing orperforming in addition a compensation of the influence of a differencebetween a frequency of a type of signals received by the first antennaand a frequency of the type of signals received by the second antenna oncarrier phase measurements.

The described apparatus or the described electronic device may furthercomprise at least one sensor providing information indicative of anorientation of the first antenna. Sensors can be used to deduce anorientation of an antenna, including direction, tilt and yaw, in aglobal coordinate system. Such at least one sensor may comprise athree-dimensional (3D) accelerometer providing an angle between theantenna plane and a vector pointing to the centre of the Earth. Such atleast one sensor may further comprise a 3D compass providing thedirection of an antenna axis in the horizontal plane. Such at least onesensor may further comprise a gyroscope, etc.

The invention can be employed for example in high-precision navigationand surveying applications. It can be provided for professional use, butalso for fun applications, such as writing with a GNSS receiver.

It is to be understood that the invention can also be employed for apositioning of more than two GNSS receivers relative to each other. Inthis case, a computational compensation could be performed for all orsome of the receivers and/or all or some of the receivers could becaused to be aligned to each other.

The invention can further be used with any kind of satellite signals, inparticular, though not exclusively, with satellite signals transmittedin a GNSS, like GPS, GLONASS, GALILEO, SBAS, QZSS, LAAS or a combinationof these. LAAS has the advantage that it enables the use of mRTK underindoor conditions as well.

It is to be understood that all presented exemplary embodiments may alsobe used in any suitable combination.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not drawn to scale and that they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram illustrating a situation with differentlyoriented GNSS antennas;

FIG. 2 is a diagram illustrating the phase response of a GPS antenna asa function of the angle of the antenna axis from the north in thehorizontal plane;

FIG. 3 is a diagram illustrating the cumulative distribution of doubledifference residuals;

FIG. 4 is a diagram illustrating the cumulative distribution of baselinelengths;

FIG. 5 is a schematic block diagram of a system according to a firstembodiment of the invention;

FIG. 6 is a flow chart illustrating an exemplary operation in the systemof FIG. 5;

FIG. 7 is a schematic block diagram of a system according to a secondembodiment of the invention;

FIG. 8 is a flow chart illustrating an exemplary operation in the systemof FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 4 illustrate the influence of a misalignment between two GNSSantennas that are used in an RTK positioning.

FIG. 1 is a diagram illustrating an exemplary constellation with two GPSsatellites 3, 4 and two receiver antennas 1, 2. The antenna axes pointto different directions. The measured carrier phases of receivedsatellite signals depend on the distance of the satellites 3, 4 from theantennas 1, 2, but in addition on the angle between a respective antennaaxis and the direction of arrival of the received signals, which areindicated in FIG. 1 with dashed lines.

As mentioned above, Real-Time Kinematics is based on solving thedouble-difference integer ambiguities. The problem formulation may leadto an exemplary measurement equation given byφ_(km) ^(pq)=ρ_(km) ^(pq) +λN _(km) ^(pq)+ε_(km) ^(pq),  (1)where φ_(km) ^(pq) is the double difference observable defined by φ_(km)^(pq)=(φ_(k) ^(p)−φ_(m) ^(p))−(φ_(k) ^(q)−φ_(m) ^(q)), where φ_(k) ^(p),φ_(m) ^(p), φ_(k) ^(q), φ_(m) ^(q) are the carrier phase measurements bythe receivers k and m of the signals originating from the satellites pand q.

Moreover, ρ_(km) ^(pq) is the difference of geometric ranges defined by

$\begin{matrix}{\rho_{km}^{pq} = {\left( {\rho_{k}^{p} - \rho_{m}^{p}} \right) - \left( {\rho_{k}^{q} - \rho_{m}^{q}} \right)}} \\{{= {\left( {{{\underset{\_}{x^{p}} - \underset{\_}{x_{k}}}} - {{\underset{\_}{x^{p}} - \left( {\underset{\_}{x_{k}} + \underset{\_}{b}} \right)}}} \right) - \left( {{{\underset{\_}{x^{q}} - \underset{\_}{x_{k}}}} - {{\underset{\_}{x^{q}} - \left( {\underset{\_}{x_{k}} + \underset{\_}{b}} \right)}}} \right)}},}\end{matrix}$where x^(p) and x^(q) are the positions of the satellites p and q,respectively. x_(k) is the position of the reference receiver and b isthe unknown baseline to be determined. Finally, λ, N_(km) ^(pq) andε_(km) ^(pq) are the wavelength, unknown double difference ambiguity(note that N_(km) ^(pq)ε

^(n×1)) and double-difference measurement noise, respectively.

It has to be noted that time variable has been dropped from theequations for the sake of clarity. However, accounting for differentmeasurement instants and time-of-flight differences between thereceivers may be required for a determination of the baseline. Further,it has to be noted that presented equation (1) represents a simplifiedform of a measurement equation, which may be modified and extended invarious ways.

Solving equation (1), which can be realized with any suitable approach,yields the fixed baseline estimate {hacek over (b)} anddouble-difference ambiguities {hacek over (N)}.

As mentioned above, using double differences has the advantage thatdouble differencing errors that are common to a given satellite signalin two receivers cancel out. Examples for such common errors comprisesatellite clock bias as well as errors induced by the troposphere andthe ionosphere.

However, additional errors are introduced by the receiving event, thatis, on the path from the surface of the antenna to the feed cable orpath.

In general, the antenna of a GNSS receiver generates an error in thephase of the signal that is dependent on the direction of the satellitewith respect to the antenna axis. This error is caused by an anisotropiccomplex frequency response of the antenna. This means that depending onthe receiving direction of the signal, a bias is induced in the carrierphase observable. If these errors are big, that is, in the order of tensof degrees, and are not compensated for, the baseline determination willfail.

FIG. 2 is a diagram presenting a fictional phase response of thereceivers 1, 2 of FIG. 1 as a function of the SV angle from the antennaaxis in the horizontal plane. An elevation dependence of the response,which would be relevant in a real application as well, has beenneglected. The maximum amplitude of the phase response can be obtainedfrom the document “GPS Antenna Design Characteristics for High-PrecisionApplications”, Journal of Surveying Engineering, Vol. 115, No. 1,February 1989, by J. M. Tranquilla and B. G. Colpitts. In this document,the authors also found that the phase response of certain antennas maydeviate as much as 60 degrees, as illustrated.

FIG. 3 is a diagram representing simulation data for different satelliteconstellations with the receiver arrangement illustrated in FIG. 1. Azero-meter baseline between the antennas 1, 2 has been simulated, whilethe phase responses of the antennas 1, 2 correspond to the phaseresponse illustrated in FIG. 2. With a baseline of zero and identicalantenna axes, there should be no double-difference residuals. In FIG. 1,however the antennas 1, 2 are arranged with differently orientated axes.FIG. 3 presents the resulting probability of double-difference residualsbetween zero and 120 degrees. The double-difference residuals show thatin the situation modeled, double difference observables may be in errorby as much as ⅓ cycle or 120 degrees, which is expected, since themaximum error is 4*60/2 degrees.

FIG. 4 is a diagram presenting the effect of the double differenceresiduals of FIG. 3 on the determined baseline. The diagram shows thecumulative distribution of baseline lengths in meters. 90% of the time,the phase error contributes an error of less than 10 cm to the baselinelength. However, significant deviations from the true length of zerometers occur also. Since the true baseline length was set to zerometers, the distribution also represents the length error in the presentcase.

The problem of double difference residuals may be solved by ensuringthat the antennas are aligned or that the effect of a misalignment iscompensated computationally. This requires information on the directionof the satellites with respect to the antenna axis and on the antennaphase response as a function of azimuth and elevation. The satellitepositions in the global coordinate system can be calculated based on theephemeredes in the received satellite signals. The phase response may bemeasured, modeled or provided by the manufacturer. The antenna axisdirection in a global coordinate system can be determined based onsensor information.

FIG. 5 is a schematic block diagram of a first exemplary system, whichallows reducing double difference residuals and thus enhancing the useof code phase measurements in accordance with an embodiment of theinvention.

The system comprises a user device 510 and a reference device 550.

The user device 510 can be for instance a mobile device, like a mobilephone.

The user device 510 comprises a processor 511 and, linked to thisprocessor 511, a memory 512, a 3D accelerometer 515, a 3D compass 516and a transceiver (TRX) 518.

The processor 511 is further linked via a GNSS signal processing unit521 to a GNSS antenna 522. GNSS signal processing unit 521 and GNSSantenna 522 form a GNSS receiver 520, which may be integrated in theuser device 510. Alternatively, though, it could also belong to anaccessory device that is connected to the user device 510. This optionis indicated in FIG. 5 by a dotted line between the GNSS receivercomponents and the other components of user device 510. A GNSS accessorydevice could be connected to the user device 510 via any suitable link,like a physical connection or a Bluetooth™ link, etc. At least in casethe GNSS receiver 520 is an accessory device that is linked to the userdevice 510 in a flexible manner, though, that is, wirelessly or by wire,the sensors 515, 516 should belong to the GNSS receiver 520 to ensurethat a detected orientation corresponds to the orientation of theantenna 522.

The processor 511 is configured to execute implemented computer programcode. The memory 512 stores computer program code, which may beretrieved by the processor 511 for execution. The stored computerprogram codes comprise a relative positioning code 513, which includesfunctional modules for orientation computations, for decisionoperations, for compensation operations and for the actual relativepositioning operations.

Obviously, the functions of the processor 511 could also be implementedin hardware in the user device 510, for example in the form of anintegrated circuit chip.

Further, it is to be understood that functions realized by the processor511 could also be realized for example by the GNSS signal processingunit 521.

The 3D accelerometer 515 is configured to detect and provide informationindicating the angle between the antenna plane and a vector pointing tothe centre of the Earth. This is information is based on theacceleration due to gravity, 9.8 ms⁻², which is also detected by anaccelerometer.

The 3D compass 516 is configured to detect and provide informationindicating the direction of the antenna axis in the horizontal plane.

The transceiver 518 enables a communication via a wireless link withanother devices. The transceiver 518 could belong for instance to acellular engine of the user device 510 and support an access to acellular communication network, or it could belong to a WLAN engine ofthe user device 510 and support an access to a WLAN, etc.

The reference device 550 can be for instance a fixed station, like abase station of a cellular communication network or a WLAN, or a networkserver linked to such a base station.

The reference device 550 comprises a processor 551 and, linked to thisprocessor 551, a memory 552, an antenna driver 555 and an interface 558.

The processor 551 is further linked via a GNSS signal processing unit561 to a GNSS antenna array 562. The GNSS antenna array 562 may beomni-directional, comprise different antennas with differentorientations or comprise movable antennas. In addition, the GNSS antennaarray 562 may comprise selectable antennas of different types. Theantenna driver 555 has moreover a controlling access to the GNSS antennaarray 562. GNSS signal processing unit 561 and GNSS antenna array 562form a GNSS receiver 560, which may be integrated in the referencedevice 550. Alternatively, though, it could also belong to a separatedevice, like a local measurement unit (LMU), that is connected to thereference device 550. This alternative is indicated in FIG. 5 by adotted line between the GNSS receiver components and the othercomponents of reference device 550. Such a local measurement unit couldbe connected to the reference device 550 using any suitable link, forexample a wired link.

The processor 551 is configured to execute implemented computer programcode. The memory 552 stores computer program code, which may beretrieved by the processor 551 for execution. The stored computerprogram codes comprise a relative positioning supporting code 553,including functional modules for antenna alignment operations and forrelative positioning related communications with a mobile device.

Obviously, the functions of processor 551 could also be implemented inhardware in reference device 550, for example in the form of anintegrated circuit chip.

The interface 558 enables a direct or indirect communication with userdevice 510. If the reference device 550 is a base station of a wirelesscommunication network, for example, the interface 558 could be atransceiver, which enables the reference device 510 to access thewireless communication network. If the reference device 550 is a networkserver, the interface may be an interface to other network elements,which connect the reference device to a base station of a wirelesscommunication network that may be accessed by the user device 510.

It is to be understood that some components of the reference device 550,like the antenna driver 555, could also belong to the GNSS receiver 560,or that functions realized by the processor 551 could also be realizedby the GNSS signal processing unit 561.

The distance and attitude between the user device and the referencedevice, or more specifically between GNSS antenna 522 and GNSS antenna562, is represented in FIG. 5 by a dashed baseline 580.

The GNSS receivers 520, 560 are both configured to operate as normalGNSS receivers. That is, they are configured to receive, acquire, trackand decode signals transmitted by satellites belonging to one or moreGNSSs, like GPS and Galileo. Further, the GNSS signal processing units521, 561 are configured to compute a stand-alone position in a knownmanner based on the received satellite signals. It is to be understoodthat the required computations could also be realized in a processingcomponent outside of the GNSS receivers 520, 560, for example inprocessor 511 or 551, respectively.

For a particular application, however, the position of user device 510might have to be determined with a high-precision. To this end, anenhanced mRTK positioning is employed, as illustrated in the flow chartof FIG. 6.

Using computer program code 513, the processor 511 of the user device510 generates in this case an initialization request, which istransmitted to reference device 550 (step 610).

Using computer program code 553, the processor 551 of the referencedevice 550 provides to the user device 510 an indication of the type ortypes of the antennas used in GNSS antenna array 562 (step 650). Inaddition, the reference device 550 could provide to the user device 510an indication of the type of signals received by the GNSS antenna array562.

The processor 511 of the user device 510 now determines whether the userdevice 510 and the reference device 550 employ identical GNSS antennas(step 611).

In case the reference device 550 provides at least one antenna which isof the same antenna type as the GNSS antenna 522, the processor 511determines an orientation of the GNSS antenna 522 in a globalco-ordinate system using information from the 3D accelerometer 515 andthe 3D compass 516 (step 620).

The processor 511 then sends a request to the reference device 550 toperform an antenna alignment and to carry out carrier phasemeasurements. The request indicates the determined orientation of GNSSantenna 522 and identifies measurement instants at which themeasurements are to be performed (step 621). The orientation only has tobe provided, though, in case the GNSS antenna array 562 is notomni-directional. In case the GNSS antenna array 562 comprises antennasof different types, the request may indicate in addition the type of theGNSS antenna 522.

Further, the processor 511 causes the GNSS signal processing unit 521 toperform carrier phase measurements at the indicated measurement instants(step 622).

The processor 551 of the reference device 550 receives the request andcauses the antenna driver 555 to align the GNSS antenna array 562,unless the GNSS antenna array 562 is not omni-directional (step 651). Analignment may be achieved, for instance, by selecting one of severalantennas of the array 562 that has a similar orientation as theindicated orientation of antenna 522. Alternatively, the alignment maybe achieved, for instance, by steering an antenna of the antenna array562 to assume an orientation corresponding to the indicated orientationof antenna 522. In case the GNSS antenna array 562 comprises antennas ofdifferent types, the antenna driver 550 further activates only anantenna of the indicated type.

Further, the processor 551 causes the GNSS signal processing unit 561 toperform carrier phase measurements at the indicated measurement instantsusing an aligned antenna of GNSS antenna array 562. The processor 551then provides the carrier phase measurements to the user device 510(step 652).

The processor 511 of the user device 510 is now able to perform relativepositioning computations using double difference observations, which areformed from the carrier phase measurements of GNSS signals received byGNSS antenna 522 and an antenna of GNSS antenna array 562 (step 612).Since the antennas have been aligned, the measurements do not containany phase offset that is caused by a difference in orientation betweenthe antennas.

Finally, the processor 511 could determine an accurate absolute positionof the user device 510, or more specifically of the GNSS antenna 522. Tothis end, the processor 551 of the reference device 550 may provide theuser device 510 in addition with a known accurate absolute position ofGNSS antenna array 552, for example together with the indication of theantenna type in step 650 or together with the carrier phase measurementsin step 652.

In case the type of the GNSS antenna 522 used by user device 510 and thetype of the antennas of GNSS antenna array 562 used by reference device550 is not the same (step 611), in contrast, the processor 511 requestsinformation from a regular GNSS positioning from the GNSS signalprocessing unit 521, including the position of the satellites from whichsignals are currently received (step 630).

Then, the processor 511 determines the orientation of the axis of theGNSS antenna 522 relative to the path of arrival of signals receivedfrom currently visible satellites using the received information on thesatellite positions and information from the 3D accelerometer 515 andthe 3D compass 516 (step 631).

Instead of new sensor information, the processor 511 could also use thepreviously determined absolute orientation of the antenna axis. Further,the processor 511 sends a request to the reference device 550 to carryout carrier phase measurements. The request identifies measurementinstants at which measurements are to be performed.

The processor 511 moreover causes the GNSS signal processing unit 521 toperform carrier phase measurements at the indicated measurement instants(step 632).

The processor 511 corrects the resulting carrier phase measurementsprovided by the GNSS signal processing unit 521 by compensating for aphase offset, which can be expected to result with the determinedrelative orientation of the antenna axis (step 633). The phase offset isdetermined from an available phase response of the antenna 522. Thephase response may be measured, modeled or provided by the manufacturer,for example in a stored look-up table. It has to be noted that theexpected phase offset will be different for each visible satellite. Anadditional parameter for adjusting the phase offset could be thedifference in frequency between the satellite signals received by theGNSS antenna 522 and the GNSS antenna array 562.

Meanwhile, the processor 551 of the reference device 550 receives therequest, causes the GNSS signal processing unit 561 to perform carrierphase measurements at the indicated measurement instants and providesthe resulting carrier phase measurements to the user device 510 (step652). Unless the GNSS antenna array 562 is omni-directional, the GNSSsignal processing unit 561 selects for these carrier phase measurementsfor each visible satellite a signal that is received by the respectiveantenna of the GNSS antenna array 562, which has the best suitedorientation. Alternatively or in addition, the processor 551 couldequally perform a computational compensation of phase offsets in thereceived signals.

The processor 511 of the user device 510 is now able to perform relativepositioning computations using double difference observations, which areformed from the carrier phase measurements of GNSS signals received byGNSS antenna 522 and GNSS antenna array 562 (step 612). Since thecarrier phase measurements provided by GNSS signal processing unit 521have been corrected in step 633, the measurement results do not containany phase offset that is caused by a difference in orientation. Finally,the processor 511 could determine an accurate absolute position of theuser device 510, or more specifically of the GNSS antenna 522.

It is to be understood that it is not required that the user device 510performs the positioning computations of step 612 itself. Alternative,it could provide its own carrier phase measurements to the referencedevice 550 for enabling the processor 551 to carry out the computations.

It may further be noted that in case both devices 510, 550 have accessto two-band receivers 520, 560, like GPS L1 and L2 receivers, it is alsopossible to use both bands in the relative positioning and to compensatefor the phase offsets between the L1 and L2 frequencies.

FIG. 7 is a schematic block diagram of a second exemplary system, whichallows reducing double difference residuals and thus enhances the use ofcode phase measurements in accordance with an embodiment of theinvention.

The system comprises again a user device 710 and a reference device 750.

The user device 710 can be again for instance a mobile device, like amobile phone or a laptop. It comprises the same arrangement ofcomponents as user device 510, including a processor 711, a memory 712,a 3D accelerometer 715, a 3D compass 716, a transceiver 718 and a GNSSreceiver with a GNSS signal process unit 721 and a GNSS antenna 722. Inaddition, a user interface (UI) is shown, which is equally linked to theprocessor 711.

The computer program codes stored in the memory 712 may comprise in thiscase a relative positioning code 713, which includes functional modulesfor orientation computations, for positioning operations, for decisionoperations, for compensation operations and for user instructions.

Obviously, the functions of processor 711 could also be implemented inhardware in user device 710, for example in the form of an integratedcircuit chip.

The reference device 750 can be as well for instance a mobile device,like a mobile phone or a laptop.

The reference device 750 comprises a processor 751 and, linked to thisprocessor 751, a memory 752, a 3D accelerometer 755, a 3D compass 756and a transceiver 758.

The processor 751 is further linked via a GNSS signal processing unit761 to a GNSS antenna 762. GNSS signal processing unit 761 and GNSSantenna array 762 form a GNSS receiver, which may be integrated into thereference device 750 or be external to the reference device 750.

The processor 751 is configured to execute implemented computer programcode. The memory 752 stores computer program code, which may beretrieved by the processor 751 for execution. The stored computerprogram codes comprise a relative positioning supporting code 753including functional modules for orientation computations and forrelative positioning related communications with another device.

Obviously, the functions of processor 751 could also be implemented inhardware in user device 750, for example in the form of an integratedcircuit chip.

The transceiver 718 of the user device 710 and the transceiver 758 ofthe reference device 750 are configured to enable a wirelesscommunication between the devices 710, 750 using a cellular link or anon-cellular link, like a wireless LAN connection, a Bluetooth™connection, a UWB connection or an infrared connection. The employedcommunication channel may also be a control plane channel or a secureuser plane location (SUPL) channel.

The GNSS receivers 721, 722 and 761, 762 are both configured to operateas normal GNSS receivers. That is, they are configured to receive,acquire, track and decode signals transmitted by satellites belonging toone or more GNSSs, like GPS and Galileo. Further, the GNSS signalprocessing units 721, 761 are configured to compute a stand-aloneposition in a known manner based on the received satellite signals. Itis to be understood that the required computations could also berealized in a processing component outside of the GNSS receivers, forexample in processor 711 or 751, respectively.

For a particular application, however, the relative position of the userdevice 710 compared to the reference device 750 might have to bedetermined with a high-precision. To this end, an enhanced mRTKpositioning is employed, as illustrated in the flow chart of FIG. 8.

Using computer program code 713, the processor 711 of the user device710 generates in this case a relative positioning request, which istransmitted to reference device 750 (step 810). The request identifiesmeasurement instants at which carrier phase measurements are to beperformed.

Moreover, the processor 711 requests information from a regular GNSSpositioning from the GNSS signal processing unit 721, including theposition of the satellites from which signals are currently received(step 811). Then, the processor 711 determines the orientation of theaxis of GNSS antenna 722 relative to the path of signals received fromcurrently visible satellites using the received information on thesatellite positions and information from the 3D accelerometer 715 andthe 3D compass 716 (step 812).

Further, the processor 711 causes the GNSS signal processing unit 721 toperform carrier phase measurements at the indicated measurement instants(step 813).

In the meantime, the processor 751 of the reference device 750 hasreceived the relative positioning request. It uses thereupon computerprogram code 753 for causing the GNSS signal processing unit 761 toperform a regular GNSS positioning (step 850). Then, the processor 751determines the orientation of the axis of GNSS antenna 762 relative tothe path of signals received from currently visible satellites using thereceived information on the satellite positions and information from the3D accelerometer 755 and the 3D compass 756 (step 851).

Further, the processor 751 causes the GNSS signal processing unit 761 toperform carrier phase measurements at the indicated measurement instants(step 852). It provides the carrier phase measurements and an indicationof the determined relative orientation for each visible satellite to theuser device 710.

The processor 711 of the user device 710 is now able to perform relativepositioning computations using double difference observations, which areformed from the carrier phase measurements of GNSS signals received byGNSS antenna 722 and GNSS antenna 762 (step 814). Since the antennas722, 762 may not have the same relative orientation for a respectivesatellite, however, the measurement results may contain a phase offsetthat is caused by the misalignment.

Therefore, the processor 711 corrects the formed double-differencesbased on the difference between the relative orientations before solvingthe integer ambiguities. The association between different misalignmentsand the required corrections may be predetermined and stored in the userdevice 710, for example in the memory 712.

In case the integer ambiguities can be solved after the compensation andthe found solution can be validated (step 815), the relative positioningis completed (step 820).

Otherwise, the processor 711 instructs the user to align the GNSSantenna 722 (step 830). The most common orientations of antennas inhandsets are vertical and horizontal, depending on the manufacturer andthe product. The instructions to the user could thus simply be “hold thedevice upright” or “lay the device down onto a horizontal surface”. Itis to be understood, however, that more differentiated instructionscould be provided as well. In addition, the processor 711 sends a newrequest to perform carrier phase measurements to the reference device750. The request includes again an indication of the desired measurementinstants.

Both processors 711, 751 then cause the respectively associated GNSSsignal processing unit 721, 761 to perform carrier phase measurements atthe indicated measurement instants (steps 831, 853).

The processor 751 provides the resulting carrier phase measurements tothe user device 710.

The processor 711 of the user device 710 is now able to perform relativepositioning computations using double difference observations, which areformed from the carrier phase measurements of GNSS signals received byGNSS antenna 722 and GNSS antenna 762 (step 832). Since the antennas722, 762 have been aligned, the phase offset in the carrier phasemeasurements that is caused by a difference in relative orientations isminimized.

Performing steps 830, 831, 853 and 832 may be tied as well to thefurther condition that GNSS antennas 722 and 762 are of the same typeand have thus the same phase response, since only this ensures than thealignment will result in a cancellation of phase response asymmetries.

The functions illustrated by the processor 511 executing program code513 or by the processor 711 executing program code 713 can be viewed asmeans for determining an orientation of a mobile first antenna based atleast on sensor information; and equally as means for causing acomputational compensation of a difference between the orientation ofthe first antenna and an orientation of a second antenna for reducing aninfluence of this difference on calculations using carrier phasemeasurements of satellite signals received by the first antenna and thesecond antenna. Alternatively, the functions illustrated by thefunctional modules of the program code 513 or the program code 713 canbe viewed as such means.

While there have been shown and described and pointed out fundamentalnovel features of the invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices and methods describedmay be made by those skilled in the art without departing from thespirit of the invention. For example, it is expressly intended that allcombinations of those elements and/or method steps which performsubstantially the same function in substantially the same way to achievethe same results are within the scope of the invention. Moreover, itshould be recognized that structures and/or elements and/or method stepsshown and/or described in connection with any disclosed form orembodiment of the invention may be incorporated in any other disclosedor described or suggested form or embodiment as a general matter ofdesign choice. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto. Furthermore, inthe claims means-plus-function clauses are intended to cover thestructures described herein as performing the recited function and notonly structural equivalents, but also equivalent structures.

What is claimed is:
 1. A method comprising: determining an orientationof a mobile first antenna based at least on sensor information;selecting one of at least two different options for reducing aninfluence of a difference between said orientation of said first antennaand an orientation of a second antenna on calculations using carrierphase measurements of satellite signals received by said first antennaand said second antenna; causing a computational compensation of thedifference between said orientation of said first antenna and saidorientation of said second antenna for reducing an influence of saiddifference on calculations using carrier phase measurements of satellitesignals received by said first antenna and said second antenna in case afirst one of said options is selected; and initiating an alignment ofsaid antennas based on said determined orientation of said firstantenna, to obtain equal orientations of said first antenna and saidsecond antenna in case a second one of said options is selected.
 2. Themethod according to claim 1, wherein said second option is selected incase said first antenna and said second antenna receiving satellitesignals are suited to be aligned and wherein said first option isselected otherwise.
 3. The method according to claim 2, furthercomprising receiving an indication of a type of said second antenna,wherein it is determined that said first antenna and said second antennaare suited to be aligned in case said first antenna and said secondantenna are of a same type and an alignment of at least one of saidantennas is supported.
 4. The method according to claim 1, wherein, ingeneral, said first option is selected and wherein said second option isselected in case a computational compensation has not been successful.5. The method according to claim 1, wherein initiating an alignmentcomprises at least one of instructing a user to align said firstantenna; and requesting another device to cause an alignment of saidsecond antenna.
 6. The method according to claim 1, wherein initiatingan alignment comprises requesting another device to cause an alignmentof said second antenna, said other device causing an alignment by atleast one of: selecting one of several available antennas as said secondantenna; steering said second antenna to assume an orientation similarto said orientation of said first antenna; and instructing a user toalign said second antenna.
 7. The method according to claim 1, furthercomprising requesting another device to select one of differentavailable antennas of different types as said second antenna such thatsaid second antenna is of a same type as said first antenna.
 8. Themethod according to claim 1, wherein determining said orientation ofsaid first antenna comprises determining said orientation in a globalcoordinate system.
 9. The method according to claim 1, whereindetermining said orientation of said first antenna comprises determiningsaid orientation relative to a path of satellite signals received bysaid first antenna.
 10. The method according to claim 1, wherein causingsaid computational compensation comprises canceling a phase offset incarrier phase measurements of satellite signals received by said firstantenna compared to carrier phase measurements of said satellite signalsreceived by said first antenna that would result with a predeterminedorientation of said first antenna.
 11. The method according to claim 1,wherein causing said computational compensation comprises canceling aphase offset in carrier phase measurements of satellite signals receivedby said first antenna compared to carrier phase measurements of saidsatellite signals received by said first antenna that would result ifsaid orientation of said first antenna was equal to said orientation ofsaid second antenna.
 12. The method according to claim 1, whereincausing said computational compensation comprises correcting carrierphase measurements of signals received by said first antenna, saidmethod further comprising transmitting said corrected carrier phasemeasurements to a device having access to carrier phase measurements ofsaid second antenna for a relative positioning between said firstantenna and said second antenna.
 13. The method according to claim 1,wherein said calculations are calculations for a relative positioningbetween said first antenna and said second antenna, said method furthercomprising performing said relative positioning calculations.
 14. Themethod according to claim 1, wherein said calculations are calculationsfor a relative positioning, said method further comprising performingsaid relative positioning calculations, wherein performing said relativepositioning comprises determining double differences from said carrierphase measurements, and wherein causing said computational compensationcomprises correcting said double differences.
 15. An apparatuscomprising a processing component, said processing component beingconfigured to determine an orientation of a mobile first antenna basedat least on sensor information; said processing component beingconfigured to select one of at least two different options for reducingan influence of a difference between said orientation of said firstantenna and an orientation of a second antenna on calculations usingcarrier phase measurements of satellite signals received by said firstantenna and said second antenna; said processing component beingconfigured to cause a computational compensation of the differencebetween said orientation of said first antenna and said orientation ofsaid second antenna for reducing an influence of said difference oncalculations using carrier phase measurements of satellite signalsreceived by said first antenna and said second antenna in case a firstone of said options is selected; and said processing component beingconfigured to initiate an alignment of said antennas based on saiddetermined orientation of said first antenna, to obtain equalorientations of said first antenna and said second antenna in case asecond one of said options is selected.
 16. The apparatus according toclaim 15, wherein said processing component is configured to select saidsecond option in case said first antenna and said second antennareceiving satellite signals are suited to be aligned and to select saidfirst option otherwise.
 17. The apparatus according to claim 16, whereinsaid processing component is further configured to receive an indicationof a type of said second antenna; and wherein said processing componentis configured to determine that said first antenna and said secondantenna are suited to be aligned in case said first antenna and saidsecond antenna are of a same type and an alignment of at least one ofsaid antennas is supported.
 18. The apparatus according to claim 15,wherein said processing component is configured to select, in general,said first option and to select said second option in case acomputational compensation has not been successful.
 19. The apparatusaccording to claim 15, wherein said processing component is configuredto initiate an alignment by at least one of: instructing a user to alignsaid first antenna; and requesting another device to cause an alignmentof said second antenna.
 20. The apparatus according to claim 15, whereinsaid processing component is further configured to request anotherdevice to select one of different available antennas of different typesas said second antenna such that said second antenna is of a same typeas said first antenna.
 21. The apparatus according to claim 15, whereinsaid processing component is configured to determine said orientation ofsaid first antenna in a global coordinate system.
 22. The apparatusaccording to claim 15, wherein said processing component is configuredto determine said orientation of said first antenna relative to a pathof satellite signals received by said first antenna.
 23. The apparatusaccording to claim 15, wherein said processing component is configuredto cause said computational compensation by canceling a phase offset incarrier phase measurements of satellite signals received by said firstantenna compared to carrier phase measurements of said satellite signalsreceived by said first antenna that would result with a predeterminedorientation of said first antenna.
 24. The apparatus according to claim15, wherein said processing component is configured to cause saidcomputational compensation by canceling a phase offset in carrier phasemeasurements of satellite signals received by said first antennacompared to carrier phase measurements of said satellite signalsreceived by said first antenna that would result if said orientation ofsaid first antenna was equal to said orientation of said second antenna.25. The apparatus according to claim 15, wherein said processingcomponent is configured to cause said computational compensation bycorrecting carrier phase measurements of signals received by said firstantenna; and wherein said processing component is further configured totransmit said corrected carrier phase measurements to a device havingaccess to carrier phase measurements of said second antenna for arelative positioning between said first antenna and said second antenna.26. The apparatus according to claim 15, wherein said calculations arecalculations for a relative positioning between said first antenna andsaid second antenna, and wherein said processing component is furtherconfigured to perform said relative positioning calculations.
 27. Theapparatus according to claim 15, wherein said calculations arecalculations for a relative positioning between said first antenna andsaid second antenna, and wherein said processing component is furtherconfigured to perform said relative positioning calculations, whereinperforming said relative positioning calculations comprises determiningdouble differences from said carrier phase measurements, and whereinsaid processing component is configured to cause said computationalcompensation by correcting said double differences.
 28. The apparatusaccording to claim 15, further comprising a sensor providing informationindicative of on an orientation of said first antenna.
 29. An electronicdevice comprising: an apparatus according to claim 15; and said firstantenna.
 30. An arrangement comprising: a first device including anapparatus according to claim 15; and a second device including saidfirst antenna.
 31. A system comprising: a first device including anapparatus according to claim 15; and a second device comprising saidsecond antenna.
 32. A computer program product in which a program codeis stored in a computer readable medium, said program code realizing thefollowing when executed by a processor: determining an orientation of amobile first antenna based at least on sensor information; selecting oneof at least two different options for reducing an influence of adifference between said orientation of said first antenna and anorientation of a second antenna on calculations using carrier phasemeasurements of satellite signals received by said first antenna andsaid second antenna; causing a computational compensation of thedifference between said orientation of said first antenna and saidorientation of said second antenna for reducing an influence of saiddifference on calculations using carrier phase measurements of satellitesignals received by said first antenna and said second antenna in case afirst one of said options is selected; and initiating an alignment ofsaid antennas based on said determined orientation of said firstantenna, to obtain equal orientations of said first antenna and saidsecond antenna in case a second one of said options is selected.
 33. Anapparatus comprising: means for determining an orientation of a mobilefirst antenna based at least on sensor information; means for selectingone of at least two different options for reducing an influence of adifference between said orientation of said first antenna and anorientation of a second antenna on calculations using carrier phasemeasurements of satellite signals received by said first antenna andsaid second antenna; means for causing a computational compensation ofthe difference between said orientation of said first antenna and saidorientation of a second antenna for reducing an influence of saiddifference on calculations using carrier phase measurements of satellitesignals received by said first antenna and said second antenna in case afirst one of said options is selected; and means for initiating analignment of said antennas based on said determined orientation of saidfirst antenna, to obtain equal orientations of said first antenna andsaid second antenna in case a second one of said options is selected.